Graft-versus-host disease (GVHD) is one of the major complications of allogeneic bone marrow transplantation (BMT). Strategies designed to limit GVHD are directed either toward the use of prophylactic or therapeutic immunosuppression, or depletion of the cells believed to mediate GVHD (mature donor lymphocytes) from the graft prior to BMT. The latter approach, through the use of lectin and E rosette fractionation techniques, monoclonal antibody and complement mediated cytolysis, or elutriation, resulted in a significant reduction in the incidence and severity of GVHD, but introduced a new and somewhat unexpected set of complications: higher rates of graft rejection, leukemic relapse, and delay in immune reconstitution. Although these complications have not manifested themselves uniformly in all studies, there is a growing body of evidence that suggests that graft-versus-host, graft-versus-leukemia, and graft-versus- rejection, as well as a significant portion of early immune reconstitution, may all be mediated, fully or in part, by mature donor lymphocytes. This proposal is directed toward improvement of allogeneic BMT for hematologic malignancies by examining the effect of lymphocyte depletion by Counterflow Centrifugal Elutriation (CCE) on GVHD, engraftment and leukemic relapse. CCE was chosen over other techniques because we believe it to be among the simplest, fastest and most reproducible methods available. More importantly, because it separates cells on the basis of physical properties (size/density), it appears well suited to graft engineering beyond lymphocyte depletion. Through integrated studies at basic, preclinical, and clinical levels, we hope to use sedimentation heterogeneity as a tool to dissect the relationships between the positive and negative effects of donor lymphocytes transferred during BMT. Proposed studies include functional and phenotypic characterization during BMT. Proposed studies include functional and phenotypic characterization of CCE separated lymphocytes with reference to the hypothesis that immunization of CCE separated lymphocytes with reference to the hypothesis that immunization may drive antigen specific cells into a "blast window", thereby causing them to coseparate with hematopoietic progenitors. CCE size heterogeneity will also be used to examine functional differences in monocyte and hematopoietic progenitor populations. Finally a phase III clinical trial will permit us to gauge the potential efficacy of CCE- lymphocyte depletion for prevention of GVHD in allogeneic BMT.